Many communication systems employ circuits or subsystems that receive multiple modulated input signals through a plurality of input ports, perform pre-determined operations, and output one or more signals via one or more output ports. In many cases, internal operations performed by the circuit or sub-system in question involve scaling and/or phase shifting the input signals and forming particular combinations of the input signals or channels to obtain a desired output. The circuit's performance in such cases is often sensitive to any unintended inter-port cross-talk and deviations in signal transfer functions within the circuit from their ideal, or target characteristics. Therefore, to achieve high performance, it is typically required to either fine-tune the circuit's internal parameters e.g. during the manufacturing or, if it is not possible or practical to do, to pre-distort input signals in a particular way adjusted to a particular circuit so as to compensate as much as possible for the circuit's non-ideality.
One example of such circuit is a multi-port amplifier (MPA), which is also referred to in the art as a hybrid matrix amplifier, and is used, for example, in multi-beam communication systems to efficiently share amplifier power among multiple communication channels or beams when the number of such channels or beams can vary, e.g. depending on capacity demands. A four-port example of such an amplifier is schematically shown in FIG. 1. It consists, essentially, of three sections: input coupler matrix (IHM) network 10 formed by a number, in this case four, preferably identical 3-dB/90° hybrid combiners 25, also referred to as 3 dB couplers; a set 15 of amplifiers (PA), one of each of the four input channels 5; and an output coupler matrix (OHM) network 20, which is also formed by a number of 3-dB/90° hybrid combiners 25, and is substantially identical to the IHM 10. The IHM splits each of the input signals 5 between the four PA, so that each output of the IHM 10 is a sum of all input signals (5) p1 to p4 with pre-determined phase shifts so that each amplifier 15 is operating on all signals. The amplifiers 15 operate preferably in their linear region and ideally have equal gain and phase shift associated therewith. The amplified signals are then fed to the OHM that phase shifts the signals in such a manner that each of the output ports 30 provides a single input port signal pi, i=1, . . . , 4, after having been amplified by all amplifiers 15, so that, for example, the output signal r1=p1, the output signal r2=p2, etc. By controlling the relative amplitude of the input signals pi, the power allocated to each signal can go from 0 to 100% of the total power available from the set 15 of the amplifiers. This allows moving power amongst channels or beams therefore enabling the move of bandwidth/capacity easily as per the traffic demand.
However, any deviation in gain/attenuation and phase shift transfer function in the couplers 25 and/or amplifiers 15 from the ideal ones would result in a distortion of the output signals, reduction of the output power of the useful signal, and signal leakage from one port to another when a signal from one of the input ports 5 appears in more than one output ports 30. When the signals share a bandwidth, the signal leakage results in channel cross-talk and thus interference, in addition to the output signal power reduction, thereby detrimentally affecting the performance of the communication link. When the input signals have no overlapping bandwidth, the cross-talk signals limit the frequency re-use capability offered by the multiple beam spatial discrimination.
It is therefore typically required to maintain the transfer function of each element of the MPA as close as possible to the ideal one in order to have a good performance from the MPA. This could potentially be accomplished by imposing tight specifications on the MPA components and the fabrication processes, which however leads to a costly system if at all achievable.
Another approach is to pre-distort the input signals such that the deviation from the ideal transfer function of the MPA is compensated. This involves an estimation of the transfer function of the MPA, which is commonly achieved through a calibration process. A typical prior-art calibration process includes an injection of a calibration signal and therefore cannot be done during a normal operation of the MPA, and thus involves an interruption of the communication link when the calibration has to be done on an installed circuit, which is highly undesirable.
Another example of a multi-port circuit wherein pre-distortion of input signals helps to achieve a better performance is a quadrature direct transmitter, which is schematically illustrated in FIG. 2. Such a transmitter may include a digital signal generator 40 to produce an in-phase (I) and a quadrature (Q) signal, two transmit chains 60 and 65 which convert the digital I and Q signals into analog signals, filter and amplify these analog signals, and a vector modulator 80 fed by the analog I and Q signals. Within the vector modulator 80, the analogue I and Q signals independently modulate in-phase and quadrature components of a carrier signal generated by a local oscillator (LO) 50. In order for the direct transmitter to perform well, the transmit chains 60, 65 must be matched in gain and phase, and their DC offsets must be as expected by the vector modulator 80. In addition, the vector modulator 80 must provide an exact 90 degrees phase shift of the LO signals received by mixers 75 and 75′, and the mixers' response must be matched in gain and phase.
These conditions are difficult to achieve, especially for vector modulators operated at microwave and higher frequencies. In practice, the vector modulator inputs are tuned, or pre-distorted, to compensate for the gain/phase imbalances, and DC offsets in the circuit. The signal tuning may consist in adjusting the relative amplitude and phase of the analogue I and Q signals and in adjusting the DC offset on both signals. Such a technique described, for example, in a U.S. Pat. No. 4,930,141, issued May 29, 1990, wherein a look-up table is used to store pre-distortion coefficients for analogue I and Q signals. Alternatively, the tuning can be done by pre-compensating the I and Q signals in the digital signal generator to achieve similar results.
However, signal pre-distortion techniques used heretofore for calibration of multi-port circuits and subsystems have some disadvantages. First, many of them require the use of specially-designed calibration signals as the circuit's input, and cannot therefore be used when the circuit is embedded in a working communication system without disrupting normal operation thereof. For example, U.S. Pat. No. 5,387,883, issued Feb. 7, 1995, describes a technique for compensating phase imbalances in a quadrature modulator using calibration signals to determine pre-distortion phase shifts. U.S. Pat. No. 5,293,406 issued Mar. 8, 1994, discloses a technique for determining pre-distortion coefficients for DC offset, gain imbalance and phase imbalance sequentially using a variety of calibration signals.
Other techniques to determine various signal pre-distortion parameters for vector modulators are described in James K. Cavers, A fast method for adaptation of quadrature modulators and demodulators in amplifier linearization circuits, Proc. Of IEEE Vehicular Technology Conference, Atlanta, Apr. 28-May 1, 1996, Vol. II, pp. 1307-1311; R. Datta, S. N. Crozier, Direct modulation at L-band using a quadrature modulator with feedback, Proc. Of the 4th Int'l Mobile Satellite Conference—IMSC'95, Jun. 6-8, 1995, Ottawa, Canada; James K. Cavers, Maria W. Liao, Adaptive compensation for imbalance and offset losses in direct conversion transceivers, IEEE Trans. On Vehicular Technology, Vol 42, No. 4, November 1993, pp. 581-588, M. Faulkner, T. Mattsson, W. Yates, Automatic adjustment of quadrature modulators, Electronics Letters, Vol. 27, No. 3, Jan. 31, 1991, pp. 214-216. Although the techniques described in these papers appear to serve their intended purposes, all of them require the use of special training or calibration signals and thus cannot be performed during normal operation of the respective transmitters.
Similarly, many prior-art techniques for determining signal pre-distortion parameters in application to multi-port amplifiers also rely on injecting test signals and therefore cannot be performed with the amplifier in operation. Examples include techniques described in U.S. Pat. No. 6,661,284 issued to Yuda Luz et al, U.S. Pat. No. 5,784,030 issued to S. O. Lane et al, and an article J. P. Starski, Calibration block for digital beam forming antenna, Antennas and Propagation Society International Symposium, Volume 4, 18-23 June 1995, Pages: 1978-1981.
Prior art techniques requiring output signal manipulation, e.g. sampling at the modulation rate or above, signal synchronization and/or frequency down-conversion: Scott A. Leyonhjelm, Michael Faulkner, The effect of reconstruction filters on direct upconversion in a multichannel environment, IEEE Trans. On Vehicular. Technology, Vol 44, No. 1, February 1995, pp. 95-102; Qiming Ren, Ingo Wolff, Improvement of digital mapping predistorters for linearising transmitters, 1997 IEEE-MTT-S proceeding, Jun. 8-13, 1997, vol. 111, pp. 1691-1694 (signal de-modulation); Rossano Marchesani, Digital precompensation of imperfections in quadrature modulators, IEEE Trans. On Comm., Vol. 48, No. 4, April 2000, pp. 552-556.
U.S. Pat. No. 6,771,709, which is issued to the inventors of the current invention and is incorporated herein by reference, describes a direct transmitter self-calibrating technique that estimates the gain/phase imbalances and DC offsets in the vector modulator and pre-compensate for their effects. It employs a nonlinear mapping between the modulator parameters and its output power to simplify the problem, and a least-squares method to estimate the modulator parameters. The technique can be used without interrupting the normal transmitter operation, and yields an excellent compensation of the gain/phase imbalance and DC offsets. However, the technique needs to relate the modulator output signal to its input signal, and an accurate synchronization between them is required to achieve a good performance, increasing the hardware cost required for its implementation. Furthermore, relatively complex digital signal processing hardware and software is required to implement the synchronization and the parameter estimation, especially at very high transmission rate.
European patent application EP 1126544A2 by S. Pietrusiak, entitled System for calibrating and characterizing an antenna system and method for characterizing an array of antenna elements, describes a process of calibrating a coupler matrix amplifier system that involves injecting a test signal and filtering out interfering signals at the output, followed by its demodulation for deriving a phase and gain transfer function of the amplifier. Drawbacks of the method include the need to inject test signals and therefore to interrupt the normal operation of the system, and the need to perform frequency conversion and demodulation of the output signal, followed by high-rate sampling thereof at least at the Nyquist rate.
Recently, the inventors of the present invention developed a method of linearizing a single-port nonlinear circuit for processing a communication signal that relies on a unique relationship between a modulation format and statistical properties of a modulated communication signal to determine signal pre-distortion information. The method, which is described in commonly owned U.S. Pat. No. 6,885,241, involves determining a cumulative statistical characteristic, or type, of the output signal of the amplifier while the amplifier carries information traffic by sampling its envelope at a relatively low rate, comparing it to an ideal statistical characteristic for the signal, and determining a non-parametric pre-distortion function for the input signal to compensate the non-linear distortions introduced by the amplifier. Advantageously, the method does not involve interruption of the communications or any complex high-speed circuitry for bit-rate signal processing. However, the method described in U.S. Pat. No. 6,885,241 is not applicable to a multi-port circuit receiving a plurality of input signals, since it does not account for cross-talk between the input signals that lead to the output signal or signals distortions.
Accordingly, the object of the present invention is to provide a method of calibrating a multi-port circuit or sub-system that can be used without interrupting a normal operation of a communication system wherein the circuit or subsystem is used, and which does not require output signal de-modulation or processing at the Nyquist rate.
Another object of the present invention is to provide a method for determining pre-distortion parameters for a multi-port circuit that can be used during a normal operation of the circuit using low-rate sampling of the output signal.
Another object of this invention is to provide a self-calibrating circuit having multiple input ports for receiving multiple modulated signals which is adaptive to time-induced and environment-induced changes of the circuit parameters, and does not require modulation-rate processing or time-domain reconstruction of the circuit's output signal or signals.
In the context of this specification, the term “circuit” is used to mean a network of elements or devices for transmitting or receiving and manipulating signals, such as microwave electrical signals, which can include one or more circuit boards and/or one or more integrated circuits such as those embodied using one or more semiconductor chips. The terms “circuit” and “sub-system” are used herein interchangeably.